Method and device for delivering anti-tachycardia pacing therapy

ABSTRACT

A method and device for delivering anti-tachycardia pacing (ATP) therapy that includes an electrode to sense cardiac signals and to deliver the therapy, sensing circuitry, electrically coupled to the electrode, to detect the tachycardia event in response to the sensed cardiac signals, and a processor to control delivery of the therapy. The processor determines whether a return cycle length generated subsequent to the delivery of the first plurality of pacing pulses is greater than a cycle length associated with the tachycardia event and less than a sum of the cycle length associated with the tachycardia event and a total prematurity associated with the first plurality of pacing pulses, and adjusts delivery of a second plurality of pacing pulses in response to the return cycle length not being greater than the cycle length associated with the tachycardia event and less than the sum of the cycle length associated with the tachycardia event and the total prematurity associated with the first plurality of pacing pulses.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 60/773,483, filed Feb. 15, 2006,entitled “DEVICE FOR DELIVERING ANTI-TACHYCARDIA PACING THERAPY”,incorporated herein by reference in its entirety.

Cross-reference is hereby made to the commonly assigned related U.S.application Ser. No. 11/675,270, entitled “METHOD AND DEVICE FORDELIVERING ANTI-TACHYCARDIA PACING THERAPY”, to Belk et al.; Ser. No.11/675,281, entitled “METHOD AND DEVICE FOR DELIVERING ANTI-TACHYCARDIAPACING THERAPY”, to Belk et al.; Ser. No. 11/675,297, entitled “METHODAND DEVICE FOR DELIVERING ANTI-TACHYCARDIA PACING THERAPY”, to Belk etal.; and Ser. No. 11/675,312, entitled “METHOD AND DEVICE FOR DELIVERINGANTI-TACHYCARDIA PACING THERAPY”, to Belk et al.; all filed concurrentlyherewith and incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method and apparatus fortreating a tachycardia event in a medical device, and, morespecifically, the present invention relates to adjusting delivery ofanti-tachycardia pacing regimens in a medical device.

A typical pacemaker/cardioverter/defibrillator (PCD) device has thecapability of providing a variety of anti-tachycardia pacing regimens.Normally, these regimens are applied according to a pre-programmedsequence, and each regimen includes a predetermined number of pacingpulses. After the series of pacing pulses is delivered, the devicechecks to determine whether the series of pulses was effective interminating the detected tachycardia. Typically, termination isconfirmed by a return to either a sinus rhythm or demand-paced rhythm,in which successive spontaneous depolarizations are separated by atleast a defined interval. If the tachycardia is not terminated, the PCDdevice delivers a subsequent series of pacing pulses having modifiedpulse parameters, e.g. reduced inter-pulse intervals and/or an alterednumber of pulses. The typical PCD device bases future treatment onwhether the tachycardia has been terminated by confirming a return tonormal rhythm. Devices which function according to the basic methodologydescribed above are disclosed in U.S. Pat. No. 4,830,006 issued toHaluska et al., U.S. Pat. No. 5,836,971 issued to Starkweather and U.S.Pat. No. 5,846,263 issued to Peterson et al.

Recent efforts have focused on modifying subsequent anti-tachycardiapacing regimens based on feedback received from previousanti-tachycardia pacing regimens. As described in U.S. Pat. No.6,167,308 issued to DeGroot, if a PCD device determines that aparticular pacing regimen, if continued, would likely not result intermination of the tachycardia, then either a new anti-tachycardiapacing regimen having modified pulse parameters is employed or thedevice delivers a high energy cardioversion pulse. To accomplish thisresult, the device first delivers a short series of pacing pulses at thedefined parameters of the pacing pulse regimen, and then interruptsdelivery of pacing pulses to await the next spontaneous depolarizationand determine the return cycle length. The device then resumes deliveryof the pacing pulse regimen for a second, greater number of pacingpulses, and again measures the return cycle length following the lastpacing pulse. In the event that no increase in the return cycle occursfollowing the delivery of the longer series of pacing pulses, the deviceterminates the pacing pulse regimen presently underway, and initiatesthe next scheduled therapy, which may be a pacing pulse regimen,(preferably at a shorter inter-pulse interval) or a cardioversion shock.Pacing pulse regimens are a preferred first treatment method fortachycardias because cardioversion shocks are more uncomfortable for thepatient and require a greater amount of energy.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and features of the present invention will be appreciated as thesame becomes better understood by reference to the following detaileddescription of the embodiments of the invention when considered inconnection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a pacemaker/cardioverter/defibrillatorand lead set of a type in which the present invention may be usefullypracticed;

FIG. 2 is a functional block diagram of circuitry located within thepacemaker/cardioverter/defibrillator of FIG. 1;

FIG. 3 is a simplified schematic view of delivery of a sequence of ATPpulses to electrode (as shown in FIG. 1) in response to a devicedetecting a tachycardia event caused by a reentrant tachycardia circuit;

FIGS. 4A-4E is a diagram of a heart with a reentrant tachycardia circuitand a pacing electrode for delivering ATP therapy;

FIG. 5 is a graphical representation of delivery of an anti-tachycardiapacing therapy according to an embodiment of the present invention; and

FIG. 6 is a flowchart of delivery of anti-tachycardia pacing therapyaccording to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows pacemaker/cardioverter/defibrillator PCD device 10, rightventricular lead 11, atrial/SVC lead 12, and coronary sinus lead 13. PCDdevice 10 delivers electrical pulses for anti-tachycardia pacing (ATP)therapy, cardioversion, and defibrillation through leads 11-13. In thepresent invention, device 10 provides customized ATP therapy forterminating reentrant tachycardia based upon analysis of the returncycle length (RCL) of a preceding train of ATP pulses. ATP therapy maybe used to terminate a reentrant tachycardia located in either theatrial or ventricular regions of the heart.

Located adjacent the distal end of right ventricular lead 11 are ringelectrode 14, tip electrode 15, and elongated coil electrode 16. At theproximal end of right ventricular lead 11 is a bifurcated connection 17,which connects electrodes 14,15, and 16 to circuitry within device 10.Electrodes 14 and 15 are used to deliver ventricular anti-tachycardiapacing (ATP) pulses, and for sensing ventricular depolarizations orR-waves, while electrode 16 is used to deliver defibrillation orcardioversion shocks.

Atrial/SVC lead 12 also includes three electrodes 18-20. Locatedadjacent the J-shaped distal end of atrial lead 12 are a second ringelectrode 18 and tip electrode 19. Located proximal to ring electrode 18is elongated coil electrode 20. At the proximal end of atrial/SVC lead12 is bifurcated connection 21, which connects electrodes 18, 19 and 20to circuitry within PCD device 10.

Electrodes 18 and 19 are used to deliver atrial ATP pulses and forsensing atrial depolarizations or P-waves, while electrode 20 is usedfor delivering defibrillation or cardioversion shocks.

Coronary sinus lead 13 includes elongated coiled defibrillationelectrode 22. Defibrillation electrode 22, illustrated in FIG. 1 as abroken line, is located within the coronary sinus and great vein of theheart. At the proximal end of coronary sinus lead 13 is connector plug23, which connects defibrillation electrode 22 to circuitry within PCDdevice 10.

Other lead configurations and electrode locations may be substituted forthe lead set illustrated. For example, in a two lead system, atrialdefibrillation and sensing electrodes might be added to either coronarysinus lead 13 or right ventricular lead 11 instead of being located onseparate atrial lead 12. In any of the configurations, all leads areconnected to circuitry within PCD device 10, which controls delivery ofATP pulses and cardioversion shocks to selected electrodes, andprocesses depolarizations sensed by the electrodes.

For the sake of simplicity, electrode 15 located at the distal end ofright ventricular lead 11 is used to illustrate delivery of ATP therapythroughout this description, although any of the electrodes discussedabove are capable of providing ATP therapy. ATP therapy delivered byelectrode 15 may be used to treat ventricular as well as atrialtachyarrhythmia, although it is usually preferable to apply ATP therapyfrom an electrode located in the chamber of origin of thetachyarrhythmia.

FIG. 2 shows a simplified functional block diagram of circuitry 24located within PCD device 10. Circuitry 24 includes pacing circuitry 25,defibrillation circuitry 26, sensor circuitry 27, control processor 28,memory 29, and communication system 30. Leads 11, 12 and 13 are eachconnected to pacing circuitry 25, defibrillation circuitry 26 and sensorcircuitry 27. This is because each lead (and in turn individualelectrodes associated with each lead) may be used in multiple capacitiesto sense depolarizations, deliver anti-tachycardia pacing pulses, anddeliver defibrillation or cardioversion shocks. Control processor 28receives input through sensor circuitry 27 from leads 11, 12 and 13concerning depolarizations sensed throughout the heart by the number ofelectrodes connected to leads 11, 12 and 13. Based on input receivedfrom sensor circuitry 27, control processor 28 performs calculations todetermine the proper course of action, which may include providing ATPtherapy to one or more electrodes through pacing circuitry 25, providingdefibrillation or cardioversion shocks to one or more electrodes throughdefibrillation circuitry 26, or providing no treatment at all. Controlprocessor 28 stores selected data to memory 29, and retrieves storeddata from memory 29 as necessary. Communication system 30 includestelemetry processor 31, transmission circuitry 32, receiving circuitry33, and antenna 34. Communication system 30 allows communication betweenPCD device 10 and devices external to the patient.

FIG. 3 is a simplified schematicview of PCD device 10 (not shown)delivering a sequence of ATP pulses 40 to electrode 15 (as shown inFIG. 1) in response to detection of a tachycardia event caused byreentrant tachycardia circuit 42. ATP pulses 40 delivered to electrode15 radiate outward from electrode 15 towards reentrant tachycardiacircuit 42. FIGS. 4A-4E show the propagation of ATP pulses 40 in greaterdetail.

FIGS. 4A-4E are schematic illustrations of termination of a reentranttachycardia using ATP therapy. FIG. 4A shows reentrant tachycardiacircuit 42 and reentrant wavefront 44 circling around circuit 42. Asreentrant wavefront 44 propagates around reentrant tachycardia circuit42, a component of reentrant wavefront 44 also radiates outward awayfrom reentrant circuit 42, shown as outward radiating reentrantwavefronts 46. Reentrant wavefronts 46 radiating away from reentrantcircuit 42 are sensed by electrode 15, which info is utilized by PCDdevice 10 to determine the presence of a tachycardia event. Thetachycardia cycle length (TCL) is a measured interval of time betweensuccessive depolarization wavefronts sensed by an electrode during atachycardia event. While a tachycardia event, which is identified inresponse to a multiple number of intervals having corresponding cyclelengths less than a predetermined threshold, and may therefore consistof a multiple number of different cycle lengths, the tachycardia eventcan be assigned a tachycardia cycle length based on a single cyclelength measurement associated with one of the multiple cycle lengths, oron an average of the cycle lengths associated with several successiveintervals.

As illustrated in FIG. 4B, PCD device 10 responds to the sensedtachycardia event by delivering ATP therapy, which consists of a numberof ATP pulses (a regimen) 48 delivered to electrode 15. ATP pulses 48radiate away from electrode 15 in all directions, including towardsreentrant circuit 42, and collide with outward radiating reentrantwavefronts 46 radiating away from reentrant circuit 42, causing acancellation of both outward radiating reentrant wavefronts 46 and ATPpulse 48. By delivering ATP pulses 48 at a rate faster than the pace ofoutward radiating reentrant wavefronts 46, collisions between ATP pulses48 and outward radiating reentrant wavefronts 46 occur further andfurther from electrode 15 and closer and closer to reentrant circuit 42.

FIGS. 4C and 4D illustrate this point, as ATP pulses 48 are provided ata rate faster than the tachycardia (and therefore the pace of incomingreentrant wavefronts 46) such that successive ATP pulses 48 in theregimen progress closer and closer to reentrant circuit 42. This processof progressing closer and closer to reentrant circuit 42 with eachsuccessive ATP pulse 48 is known as “peel back.”

In FIG. 4E, ATP pulses 48 are delivered at a rate greater than the rateof the tachycardia event that enables ATP pulses 48 to reach and enterreentrant circuit 42. With the proper timing of the ATP pulses 48,excitable cardiac tissue within reentrant circuit 42 is depolarized,thus blocking circulating of reentrant wavefront 44 within reentrantcircuit 42 and terminating the tachycardia event. As described below,during delivery of an antitachycardia pacing regimen, the presentinvention takes into account a transit time (t_(transit)), which is theamount of time for a pulse to travel from the pacing site (e.g.electrode 15) to reentrant circuit 42. Although the transit timet_(transit) could also be expressed as a distance, it is easier to thinkabout the distance between the pacing site and reentrant circuit 42 interms of time, as timing is directly controlled by the system, andbecause other values such as prematurity, described below, are expressedin time. The present invention adjusts the delivery of an ATP therapy bydetermining a transit time associated with the sensed tachycardia eventand, based on the determined transit time, determining a number ofpulses required for the ATP therapy to reach the associated reentrantcircuit, described below in detail.

FIGS. 4A-4E illustrate the basics of how ATP therapy terminatesreentrant tachycardias. In order to be effective, the ATP therapyregimen must succeed in two objectives. The first objective is toadvance the wavefront created by ATP pulses 48 all the way to thelocation of reentrant circuit 42. In order to move each successivecollision between ATP pulses 48 and outward radiating reentrantwavefronts 46 closer and closer to reentrant circuit 42, each ATP pulseprovided in a given regimen is provided at a rate, known as the ATPcycle length, that is faster than the rate of the tachycardia. The ATPcycle length is defined for each ATP pulse in a given regimen as theinterval of time from either the last sensed depolarization wavefront(for the first pace of a therapy regimen), or the previous delivered ATPpulse, until the delivery of the next ATP pulse.

The second objective is to eliminate the excitability in reentrantcircuit 42 so the reentry process can no longer occur. This is afunction of advancing the rate of the tachycardia within reentrantcircuit 42 to a rate that cannot be sustained by the reentrant circuit42 and is therefore related to the fastest pacing rate to reachreentrant circuit 42.

FIG. 5 is a graphical representation of delivery of an anti-tachycardiapacing therapy according to an embodiment of the present invention.Depolarizations or signals sensed by electrode 15 are labeled S₁, S₂,S₃, S₄, S₅, and S₆. ATP pulses in a first ATP regimen 52 delivered byelectrode 15 are labeled ATP₁, ATP₂, ATP₃ . . . ATP_(N1), with ATP_(N1)representing the last pacing pulse provided in the first regimen. ATPpulses in a second ATP regimen 54 delivered by electrode 15 are labeledATP₄, ATP₅, ATP₆ . . . ATP_(N2), with ATP_(N2) representing the lastpacing pulse provided in the second regimen. The second ATP regimen 54is only provided if the device 10 determines that the first ATP regimen52 failed to terminate the tachycardia.

The time interval between successive sensed signals S₁ and S₂ is thetachycardia cycle length (TCL₁), and alerts PCD device 10 to the ongoingtachycardia. TCL₁ is stored by PCD device 10 for a number of reasonsdiscussed below. In response to the detected tachycardia event, PCDdevice 10 generates ATP regimen 52 including ATP pulses ATP₁, ATP₂, ATP₃. . . ATP_(N1) and delivers ATP regimen 52 to an electrode (in thisexample, electrode 15), according to the embodiments described below.

According to the present invention, a prematurity of each ATP pulse(P_(i)), corresponding to how much faster the rate at which the ATPpulse is delivered relative to the rate of the tachycardia event, isdetermined and subsequently utilized to determine the number of pacingpulses to be delivered, described below. Stated differently, theprematurity of the ATP pulse (P_(i)) corresponds to how much shorter thecycle length of the ATP pulse (P_(i)) is relative to the tachycardiacycle length TCL. Therefore, the prematurity of the ATP pulse (P_(i))can be defined as the difference between that pulse's ATP cycle length(T_(i)) and the tachycardia cycle length TCL, as set forth in Equation1.P _(i) =TCL−T _(i)  Equation 1

The total prematurity of a pacing regimen (P_(TOTAL)) is defined as thesum of the individual prematurities for each ATP pulse P_(i)

$\left( {{i.e.},{P_{TOTAL} = {\sum\limits_{i = 1}^{n}\; P_{i}}}} \right).$From this definition, it can be seen that P_(TOTAL) describes theeffects of both the ATP cycle lengths and the number of ATP pulsesdelivered.

A “burst” ATP regimen is defined as an ATP regimen where all of the ATPcycle lengths T_(i) are equal, resulting in all correspondingprematurities P_(i) being equal per Equation 1. For burst ATP regimens,prematurity of each pulse is denoted simply as prematurity P. This is acommon and simple form of ATP therapy that is instructive because itgives a total prematurity P_(TOTAL) equal to the number of pulses n inan ATP regimen times the prematurity P associated with each ATP pulse(i.e., n*P).

According to the present invention, ATP pacing regimen 52 is defined byprematurity P_(i) associated with each ATP pulse and the number ofpulses n included in the regimen. In the first ATP regimen 52, ATP₁ isdelivered a time T_(i) after depolarization S₂ is sensed by electrode 15(i.e., ATP cycle length of pulse ATP₁ is T₁). The prematurity associatedwith the first pulse ATP₁ is shown in FIG. 5 as P₁=TCL₁−T₁. For example,if the tachycardia cycle length TCL was measured to be 400 ms, and ATP₁is provided 350 ms after S₂, then prematurity P₁ would be equal to 50ms, meaning that ATP₁ is provided 50 ms before the next reentrantwavefront would have been sensed by electrode 15. The next pulse ATP₂ isdelivered a time T₂ following delivery of the previous pulse ATP₁ (i.e.,ATP cycle length of pulse ATP₂ is time T₂). The prematurity associatedwith ATP₂ is P₂=TCL₁−T₂. Likewise, the prematurity associated with ATP₃delivered a time T₃ after ATP₂ is P₃=TCL₁−T₃.

According to the present invention, in order to calculate the totalprematurity P_(total) associated with ATP regimen 52, each individualprematurity (P₁, P₂, P₃ . . . P_(N1)) is summed. As discussed above, forthe sake of simplicity, the remainder of ATP pacing regimens discussedin FIG. 5 are burst pacing regimens, (i.e., constant ATP cycle length,T₁=T₂=T₃=T_(n)) making associated prematurities P_(i) equal andP_(TOTAL) equivalent to n*P. In other embodiments, different ATP pacingregimens are used (for example, ramp ATP in which the time betweensuccessive ATP pulses is shortened), and P_(TOTAL) has to be calculatedas a sum of each prematurity P_(i), making operations such as selectionof number of pacing pulses n more involved, but with the same principlesstill applying.

After delivery of the last ATP pulse (ATP_(N1)) in ATP regimen 52,electrode 15 is used to sense the next depolarization S₃. The timebetween delivery of last ATP pulse ATP_(N1) and the sensing ofdepolarization S₃ is return cycle length RCL₁ resulting subsequent todelivery of ATP regimen 52. In the present invention, return cyclelength RCL is used to provide information regarding the effect of thepreceding train of ATP pulses on reentrant tachycardia circuit 42 (shownin FIGS. 4A-4E), this information then guides subsequent therapies, aswill be described below. Device 10 stores and processes return cyclelength RCL₁, and based on a number of calculations employing returncycle length RCL, device 10 customizes subsequent ATP regimen 54. Itshould be noted that in other embodiments, more than one electrode maybe used to sense depolarizations and deliver ATP pulses. If more thanone electrode is used, then time differences caused by the physicalseparation of the two electrodes must be taken into account whenanalyzing return cycle length.

Assuming ATP regimen 52 did not terminate the reentrant tachycardia,determined by monitoring the time interval between subsequentdepolarizations (S₃ and S₄ for example), then PCD device 10 delivers ATPregimen 54 to electrode 15. Based on the return cycle length RCL₁measured following ATP regimen 52, PCD device 10 customizes ATP regimen54 to improve the likelihood that ATP regimen 54 will terminate thetachycardia. Like ATP regimen 52, to calculate the prematurityassociated with ATP regimen 54, the prematurity associated with each ATPpulse (P₄=TCL₂−ATP₅, P₅=TCL₂−ATP₆ . . . P_(N)=TCL₂−ATP_(N2)) must betaken into account. The delivery of ATP regimen 52 has no effect on thecalculation of the prematurity associated with ATP regimen 54, alltherapy regimens operate nearly independent as far as effects ontachycardia circuit 42. After delivery of the final pulse ATP_(N2) ofthe subsequent ATP regimen, device 10 measures the time until the nextsensed depolarization S₆ to determine the return cycle length (RCL₂).The process is repeated, with a subsequent pacing regimen (not shown)provided by PCD device 10 depending on whether the tachycardia has beenterminated and on the measurement of previous return cycle lengths RCL₂,and so forth.

According to the present invention, following delivery of ATP regimen 52and ATP regimens in general, there are five possible scenarios thatresult from ATP application:

-   1. Successful termination of the tachycardia;-   2. Displacement of tachycardia circuit (i.e., termination of the    original tachycardia but initiation of a new tachycardia);-   3. Failed termination because of a failure to consistently capture    tissue around the stimulation electrode (e.g., electrode 15);-   4. Failed termination because of failure to complete peelback-   5. Failed termination because of failure to extinguish reentrant    circuit excitability when circuit was reached (also know as    continuous resetting of reentrant circuit 64 or entrainment).

In the present invention, the methods for detecting which of thesepossible cases actually occurred leads to the tailoring of subsequenttherapy. Termination of reentrant tachycardia is detected by measuring areturn to a normal beating pattern of the heart. In this scenario, nofurther ATP regimens are required. Displacement is a scenario equivalentto the original initiation of a new tachycardia. For the remainingscenarios of ATP failure, device 10 uses circuitry to analyze the returncycle length RCL₁ to determine which of the remaining three scenarioshas occurred following delivery of ATP regimen 52. By determining whichscenario has occurred, device 10 is able to customize subsequent pacingregimens to better terminate the reentrant tachycardia.

As described above in reference to FIG. 5, the return cycle length (RCL)is defined as the time interval from the delivery of the final ATP pulseof a regimen to an electrode (i.e., the pacing site) until the firstdepolarization wavefront is sensed at the pacing site. According to thepresent invention, as illustrated in FIG. 4E, since a path of an ATPpulse that successfully reaches reentrant circuit 42 travels for transittime t_(transit), and enters and entrains reentrant circuit 42 (takingan amount of time equal to the tachycardia cycle length TCL), the returncycle length RCL can be expressed as a sum of the tachycardia cyclelength and the transit time t_(transit) as set forth in the followingEquation 2:RCL=TCL+2*t _(transit)  Equation 2

Solving for the transit time t_(transit) results in the followingEquation 3:

$\begin{matrix}{t_{transit} = \frac{{RCL} - {TCL}}{2}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

According to the present invention, since both the return cycle lengthRCL and the tachycardia cycle length TCL are known subsequent to thedelivery of the initial pacing regimen, the device 10 varies the nextpacing therapy that is subsequently delivered by updating the transittime t_(transit) using Equation 3, and setting the current transit timet_(transit) equal to the updated transit time t_(transit) so that thenext pacing regimen is delivered using the updated transit timet_(transit). In particular, for example, the updated transit timet_(transit) will result in the predetermined number of pacing pulses (n)being reduced, as will be described below.

FIG. 6 is a flowchart of delivery of anti-tachycardia pacing therapyaccording to an embodiment of the present invention. In order toillustrate specific application of the methods for tailoring therapy, anexample of controlling burst ATP will be carried through the descriptionof FIG. 6. In this embodiment, electrode 15 is used to detect electricalpulses for device 10, as well as for delivering ATP pulses from device10.

As illustrated in FIG. 6, according to an embodiment of the presentinvention, device 10 determines that a tachycardia event has beendetected, Block 300, in response to a measured time interval betweensuccessive depolarizations sensed by electrode 15 being less than apredetermined threshold value, for example. Once a tachycardia event isdetermined to be occurring, device 10 determines an initial tachycardiacycle length (TCL_(initial)), Block 302, based on a mean time intervalbetween successive depolarizations sensed by electrode 15. The device 10then determines a transit time t_(transit), Block 303, associated withthe detected tachycardia event.

Since the return cycle length is not known prior to the initial deliveryof pacing therapy, rather than using Equation 3 which requires a knownreturn cycle length RCL, the initial transit time t_(transit) is set asan estimate of a maximum time required to depolarize any point in theventricle from electrode 15. For example, the initial transit timet_(transit) is selected to be around 180 ms. In other embodiments, theinitial estimate of the transit time t_(transit) is selected based onpast t_(transit) times recorded by PCD device 10 or a population averagevalue for the transit time t_(transit). In other embodiments, thetransit time t_(transit) is selected based on a comparison of measuredproperties of the current tachycardia with measured values of previoustachycardias with known or estimated t_(transit) times. Tachycardiaswith similar measured properties may be indicative of similarly locatedreentrant circuits. Measured properties of tachycardias includetachycardia cycle length, as well as tachycardia complex morphology.

Device 10 also determines an initial ATP pulse period (T₁), based on theinitial return cycle length TCL_(initial), and the type of pacing schemeto be utilized for determining the ATP pulse period T_(i) of thesubsequent pulses in the initial ATP regimen, Block 304. For example, aburst, a ramp, or some other desired pacing scheme may be selected. Aprematurity associated with the initial ATP pulse period T₁ is alsodetermined in Block 304 using Equation 1. Selecting the initial ATPpulse period T_(i) has the effect of defining prematurity P_(i)associated with each ATP pulse. For example, if the initial tachycardiacycle length TCL_(initial) is measured to be 400 ms, and ATP pulseperiod T₁ is set at 360 ms (80% of TCL_(initial)), then the prematurityP₁ (TCL_(initial)−T₁) associated with ATP period T₁ is 40 ms.

Device then determines the number of ATP pulses n that are required tobe included in the pulse regimen, Block 306, based on the choice of ATPregimen pacing scheme, to ensure peelback is completed and the deliveredpacing pulse wavefronts of the pacing regimen reach reentrant circuit42. This is done by ensuring the sequence of pulses is constructed suchthat total prematurity P_(TOTAL) is at least twice time t_(transit). Forthe burst ATP example, with the initial estimated transit timet_(transit) of 180 ms, PCD device 10 uses the following Equation 4 inorder to determine the expected number of pulses n needed to reachreentrant tachycardia circuit 42. If transit time t_(transit) is knownfrom RCL analysis for the tachycardia event, i.e., determined usingEquation 3, then this known value is used in subsequent returns to Step306, as will be described below.

$\begin{matrix}{n = {\frac{P_{TOTAL}}{P} = \left\lbrack \frac{\left( {2*t_{transit}} \right)}{P} \right\rbrack}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Therefore, in this burst pacing example, if the tachycardia cycle lengthTCL is equal to 400 ms, and ATP pulse period T₁ is set at 360 ms (80% ofTCL_(initial)), so that the prematurity P₁ (TCL_(initial)−T₁) associatedwith ATP period T₁ is 40 ms, the number of pulses n is determined to beapproximately equal to 9 (n=360/40).

When a burst pacing scheme is utilized, calculation of the number ofpulse n is simplified, since all of the associated ATP pulses periodsT_(i) of the delivered pacing therapy are equal and therefore all of theprematurities P_(i) are equal. Therefore, the calculation of theprematurity P in Equation 4 involves a single calculation. If a ramp ATPregimen is utilized, example, the device 10 determines the number ofpulses n required by calculating multiple prematurities in order todetermine when the prematurity associated with ramped pulses initiallybecomes greater than or equal to twice the initial estimated transittime t_(transit). For example, assuming the initial return cycle lengthTCL_(initial) is determined to be 400 ms in Block 302, and the pulsepacing scheme is a ramp scheme in which the initial ATP pulse period T₁is set at 360 ms (80% of TCL_(initial)) and the pulse rate is increasedby 10 ms for each subsequently delivered pacing pulse, the prematurityP₁ of initial pacing pulse T₁ is 40 ms, which is not greater than orequal to twice the initial estimated transit time t_(transit) (360 ms),the prematurity P₂ of the next pacing pulse T₂ is 90 ms (40+(40+10)),the prematurity P₃ of the next pacing pulse T₃ is 150 ms, theprematurity P₄ of the next pacing pulse T₄ is 220 ms, the prematurity P₅of the next pacing pulse T₅ is 300 ms, and the prematurity P₆ of thenext pacing pulse T₆ is 390 ms. Therefore, the number of pacing pulsesrequired in such a ramp pacing scheme is approximately 6, since it is atthe 6^(th) pacing pulse T₆ that the prematurity associated with rampedpulses is first greater than or equal to twice the initial estimatedtransit time t_(transit).

Once the number of pacing pulses n has been determined, device 10delivers the pacing regimen to electrode 15, Block 308, using thedetermined pulse pacing scheme, ATP pacing periods T_(i), and the numberof pulses n.

Once the pacing regimen has been delivered, device 10 measures andstores a return cycle length RCL detected by electrode 15, Block 310.Return cycle length RCL is the measured time between the last ATP pulsedelivered by device 10 to electrode 15 and the next depolarizationsensed by electrode 15 as shown in FIG. 5.

At approximately the same time, device 10 measures time intervalsbetween successive depolarizations sensed by electrode 15 to determineif the tachycardia event has been terminated, Block 312. If device 10determines the tachycardia event has been terminated, No in Block 312,then ATP treatment is terminated, Block 314.

If tachycardia has not been terminated, Yes in Block 312, treatmentcontinues with device 10 measuring the current tachycardia cycle lengthTCL_(current), Block 316. Based on the determined current tachycardiacycle length TCL_(current), device 10 determines whether the reentrantcircuit 42 associated with the initially detected tachycardia event hasbeen displaced as a result of the delivered ATP regimen, Block 318.According to an embodiment of the present invention, device 10determines whether the tachycardia event has been displaced by detectingwhether a substantial alteration in the characteristics of thetachycardia has occurred. For example, according to one embodimentdevice 10 compares the current tachycardia cycle length TCL_(current)with the initial tachycardia cycle length TCL_(initial). If thedifference between the current tachycardia cycle length TCL_(current)and the initial tachycardia cycle length TCL_(initial) is greater than apredetermined displacement threshold, device 10 determines thatreentrant circuit 42 has been displaced, and therefore the currentdetected tachycardia event is associated with a new reentrant circuit ina new location. According to an embodiment of the present invention,displacement threshold is set as 50 ms.

If the tachycardia event has been displaced, Yes in Block 318, thereturn cycle length RCL cannot be used to provide complete informationabout the new reentrant circuit location and the situation becomesfunctionally identical to the situation prior to the first ATP regimenwhere the initial transit time t_(transit) must again be estimated,Block 303, without the use of Equation 4. In other embodiments, a changein the properties of the depolarization signal sensed by an electrode isused alone or in conjunction with a change in the current tachycardiacycle length TCL_(current) to detect displacement (e.g., tachycardiacomplex morphology). For each discovery of a new circuit, the functionalstatus returns to that of initial detection of a tachycardia, andproceeds from Block 303.

Once device 10 determines that reentrant circuit 42 has not beendisplaced, No in Block 318, device 10 uses the return cycle length RCL,Block 320, to determine the effectiveness of the previous delivered ATPtherapy. Based on the measured return cycle length RCL, device 10 isable to distinguish between three remaining scenarios that are possiblefollowing delivery of an ATP pacing regimen: (1) failure to capture, (2)failure to complete peelback, and (3) reset of reentrant circuit but notermination. The following return cycle length RCL relationships (inwhich TCL=TCL_(initial)) identify the scenarios.

Scenario F1: RCL ≦ TCL Failure to Capture Scenario F2: RCL > orIncomplete Peelback approximately = TCL + P_(TOTAL) Scenario F3: TCL <RCL < TCL + P_(TOTAL) Reset

In particular, in order to determine the effectiveness of the deliveredATP therapy, device 10 determines whether the return cycle length RCL isless than or equal to the tachycardia cycle length TCL, prior to thedelivery of the therapy, Block 321. If the return cycle length RCL isless than or equal to the tachycardia cycle length TCL, Yes in Block321, the delivered therapy is determined to have failed to capture thetachycardia event, Block 322.

According to the present invention, such a determination of failure tocapture Block 321 being detected is indicative that the pacing period ofone or more pulses of the prior pacing regimen was likely too fast,preventing cardiac tissue surrounding electrode 15 from recovering fromthe previous pacing pulse. Therefore, in response to determining thatthe delivered therapy failed to capture the tachycardia event, Block322, device 10 changes the ATP pacing therapy by increasing at least oneof the ATP pacing periods T_(i) associated with the next ATP therapy tobe subsequently delivered, in order to allow more recovery time for thecardiac tissue. That is, the coupling interval between the pulse thatfailed to capture and the previous pulse is increased to provide moretime for tissue to recover. The result of increasing the at least one ofthe ATP pacing periods T_(i) is that the prematurity P of Equation 4 isdecreased, Block 324.

According to an embodiment of the present invention, device 10 increasesall of the periods T_(i) when burst pacing is utilized, for example,resulting in all of the associated prematurities P_(i) being decreased.Since decreasing the prematurities P_(i) of each of the ATP pulsesrequires that the number of pulses n delivered must be increased inorder to provide the total prematurity P_(TOTAL) necessary to completepeelback, once device 10 determines the delivered ATP therapy failed tocapture reentrant circuit 42, device 10 updates the prematurity P, i.e.,decreases the prematurity P by increasing the ATP pacing periods T_(i),Block 324. Device 10 then updates the number of pulses n required inorder to reach reentrant circuit 42 using Equation 4 and newly decreasedprematurity P, based on the current value of time t_(transit), eitherusing the estimated initial value or using an updated value determinedusing Equation 3 and the return cycle length RCL determined in Block 310and the tachycardia cycle length TCL determined in Block 316 from aprior delivered ATP sequence. For the burst example, as ATP period T_(i)is increased (to prevent a second pacing regimen from failing tocapture) the number of pulses required to reach reentrant circuit 42increases per Equation 4. After selecting a new pacing period T_(i),resulting in a decrease in prematurity P, Block 324, and determining thenumber of ATP pulses n required to reach reentrant circuit 42, Block306, device 10 delivers the new ATP regimen, Block 308.

After delivering the new ATP pulse train, Block 308, device 10 againmeasures the return cycle length RCL, Block 310, while also measuringsuccessive sensed depolarizations to determine if the tachycardia eventhas been terminated, Block 312. If the tachycardia event has not beenterminated, then device 10 uses the new return cycle length RCL, Block316, to determine the effect of the latest ATP pulse train on reentrantcircuit 42, Block 320, and update the subsequent delivery of ATP therapybased on the determined effect.

If device determines that the return cycle length is not less than orequal to the tachycardia cycle length TCL, No in Block 321, andtherefore there was not a failure to capture, device 10 determineswhether the return cycle length RCL is greater than or equal to the sumof the tachycardia cycle length TCL and the total prematurity P_(TOTAL),Block 326. If the return cycle length RCL is determined to be greaterthan the sum of the tachycardia cycle length TCL and the totalprematurity P_(TOTAL), Yes in Block 326, the effect of peelback was notcomplete and therefore the delivered therapy is determined to havefailed to reach reentrant circuit 42, Block 330.

If a burst pulse therapy regimen is utilized, for example, each of theindividual prematurities are equal, and therefore the return cyclelength RCL is determined to be greater than the tachycardia cycle lengthTCL plus the product of the number of pulses delivered n and theprematurity P_(i) associated the pulses (RCL>=(TCL+n*P)) for ScenarioF2. This scenario (Block 330) of incomplete peelback indicates that thetotal prematurity P_(TOTAL) of the ATP regimen was insufficient.Therefore, according to an embodiment of the present invention, device10 increases the total prematurity P_(total) of the next ATP regimen,Block 332, by increasing the prematurity P_(i) of one or more pulses, orincreasing the number of pulses n or some combination of the two. In oneembodiment, the number of ATP pulses is increased by 50% and theprematurity P_(i) associated with each ATP pulse is kept the same. Thenew pacing regimen is then delivered in Block 308, and the processcontinues as described above.

In scenario F2, the actual transit time t_(transit) between electrode 15and reentrant circuit 42 cannot be determined from the return cyclelength RCL subsequent to the delivered ATP sequence because those ATPpulses never reached reentrant circuit 42. However, a minimum transittime t_(transit) can be established based on the failure of the pacingregimen to reach reentrant circuit 42, where, for example, the minimumis at least one-half the total prematurity P_(TOTAL) of the ATPsequence. This information is stored for future reference in determiningthe minimum number of pacing cycles initially required to reachreentrant circuit 42, both within the same tachycardia and applied tofuture tachycardias at Block 306 as previously discussed.

If device 10 determines that the return cycle length RCL is both notless than or equal to the tachycardia cycle length TCL, No in Block 321,and not greater than or equal to the sum of the tachycardia cycle lengthTCL and the total prematurity P_(TOTAL), No Block 326, the device 10determines that the return cycle length must therefore be greater thanthe tachycardia cycle length TCL but less than the sum of thetachycardia cycle length TCL and the total prematurity P_(TOTAL), Block334, and therefore concludes that the most recently delivered pacingregimen reached reentrant circuit 42 but failed the to terminate thetachycardia, Block 335, i.e., the failure scenario is Scenario F3. Thus,the previous ATP regimen reached reentrant circuit 42 and entrainedreentrant circuit 42, but it did not successfully terminate thetachycardia event occurring in reentrant circuit 42. Once device 10determines it has reset reentrant circuit 42, Block 335, device 10determines the transit the time t_(transit) from electrode 15 to theentrance to reentrant circuit 42 using the return cycle length Equations2 and 3, Block 336. The transit time t_(transit) solved for in Equation3 is the actual travel time from electrode 15 to reentrant circuit 42,not an estimated time. Equation 2 describes the path of an ATP pulsethat successfully reaches reentrant circuit 42 (traveling for transittime t_(transit)), enters and entrains reentrant circuit 42 (taking anamount of time equal to TCL), and then travels back to electrode 15(traveling for time t_(transit)). This value is stored and identifiedwith the particular characteristics of the tachycardia (e.g., TCL,electrical signal characteristics sensed by an electrode, or combinationof both), Block 337. Device 10 may use stored t_(transit) times toestimate future t_(transit) times (e.g., at Block 303) associated withsubsequent tachycardias. In addition, if the embodiment uses a maximumestimate of transit time t_(transit) to select a number of pulses n inBlock 306, the maximum estimate of transit time t_(transit) is updatedto the measured transit time t_(transit) if the measure is longer.

After determining transit time t_(transit), device 10 crafts acustomized ATP sequence in Block 338. For example, device determines theminimum number of ATP pulses n required to complete peelback and reachreentrant circuit 42 in the same manner as in Block 306, but using thedetermined transit time t_(transist). For the burst ATP example this canbe done using Equation 4, in which P_(i)=P.

In general, once transit time t_(transit) is measured via a return cyclelength RCL from a reset scenario, device 10 can alter ATP pacing periodT_(i) and still calculate conclusively the number of ATP pulses nrequired to reach reentrant circuit 42 at the new pacing period.Typically, device 10 will generate at least one ATP pulse more thanrequired to reach reentrant circuit 42. In addition, because returncycle length RCL indicates the prior therapy did interact with thetachycardia, it is determined that none of the prematurities P_(i) towhich reentrant circuit 42 was exposed were large enough to terminateexcitability. Thus, the other step in customization is to produce asequence where the pulses that reach reentrant circuit 42 produce alarger prematurity P than has been delivered in the past. In oneembodiment, the prematurities P_(i) of the previous ATP sequence arekept constant, the number of pulses n is selected to be one greater(n+1) than the number needed to reach reentrant circuit 42 based on theresult of Equation 4, then an additional pace is provided at a moreaggressive (higher) individual prematurity P_(i). For the burst exampleunder discussion, all prematurities P_(i) are kept constant. Therefore,the delivery of a larger prematurity P_(i) to the circuit raises all ofthe prematurities P_(i). For burst customization, a larger prematurity Pis selected, this is applied to the determined the tachycardia cyclelength TCL to determine a new ATP pacing period T_(i) for the burstsequence and via Equation 4 to determine the necessary number of pulsesn to ensure interaction with reentrant circuit 42. After selecting thenumber of pulses n required and the cycle lengths T_(i) (thus fullyspecifying the ATP regimen), Block 338, the customized ATP regimen isdelivered, Block 308, and the process continues as described previously.

The example operation of FIG. 6 is only one of a variety of responsesthat RCL analysis makes possible. Application of the fundamentalinformation discovered via the application of return cycle length RCLanalysis of the invention allows more complex analysis incorporating ahistory of return cycle length RCL or transit time t_(transit). As anillustrative example, the analysis to determine failure to capture isexpanded to incorporate additional cases beyond return cycle length RCLand tachycardia cycle length TCL.

In this embodiment, at Block 320, an additional analysis identifiesalternate manifestations of Scenario F1 (failure to capture). Forinstance, if the last ATP pulse (ATP_(N)) in the ATP regimen fails tocapture, this could be indicative that the tissue surrounding electrodehad not yet recovered (repolarized) from the previous ATP pulse(ATP_(N-1)) delivered. The result is that ATP pulse ATP_(N) does notpropagate towards reentrant circuit 42. This is typical in ATP pulsingregimens in which the last pulse is provided at a more aggressive pace(i.e., has a larger prematurity P than previous pulses). To determine ifthe last pulse failed to capture, PCD device 10 compares the returncycle length RCL associated with the current ATP regimen with anexpected return cycle length RCL (RCL_(expected)). If a previousunsuccessful series resulted in the determination of transit timet_(transit) for the tachycardia, then the expected return cycle lengthRCL_(expected) for a series is approximately 2* t_(transit)+TCL.Otherwise, device 10 can estimate the expected return cycle lengthRCL_(expected) to some extent based on the relationship between totalprematurity P_(TOTAL) of the current ATP regimen and return cycle lengthRCL (i.e., as total prematurity P_(TOTAL) increases, return cycle lengthalso increases in linear fashion). This relationship gives an expectedreturn cycle length RCL_(expected) equal to TCL+P_(TOTAL). Thus, device10 can approximate, based on the total prematurity associated with thecurrent ATP regimen and the expected return cycle length RCL.

Once the expected return cycle length RCL_(expected) has been calculatedby one of the method discussed above, it is compared with the currentreturn cycle length RCL_(current). If the current return cycle lengthRCL_(current) is equal to the difference between the expected returncycle length RCL_(expected) and the pacing period (T_(N)) of the lastATP pulse (ATP_(N)) in the ATP regimen (i.e.RCL_(current)=RCL_(expected)−T_(N)), then device 10 determines that thelast ATP pulse (ATP_(N)) most likely failed to capture, but previouspulses were able to capture. In this case, device 10 lowers theprematurity P_(N) of the last ATP pulse (ATP_(N)) in the ATP regimen andthen delivers the pacing regimen (with less aggressively paced ATP_(N)),Block 308. In another embodiment, not only is prematurity P_(N) of thelast ATP pulse decreased, but an additional stimulus is added at theoriginal prematurity P_(N) of the prior sequence. In another embodiment,the prematurity P_(N-1) of the penultimate pulse of the new ATP regimenis set to be less than the prematurity of the pulse that failed tocapture. The prematurity P_(N) of the final pulse is set to be greaterthan the prematurity P_(N-1) of the penultimate pulse. In theseembodiments, the reduced prematurity of the penultimate pulse conditionsthe local tissue such that the final pulse is more likely to capture.

According to the present invention, device 10 selects, based ondetection of a tachycardia event, an initial number of pulses based onan estimated distance (i.e., transit time) between an electrode and areentrant circuit located somewhere in the heart. After applying aninitial ATP regimen, device 10 measures return cycle length RCL todetermine the effect of the initial pacing regimen on the reentranttachycardia. Device 10 uses return cycle length RCL to determine theeffect of the previous pacing regimen on the tachycardia, i.e. failureto capture generally, failure to capture last pulse, failure to completepeelback, or resetting of reentrant circuit. Based on the return cyclelength RCL, device 10 alters subsequent ATP regimens to more effectivelyterminate the reentrant tachycardia. Termination of the reentranttachycardia is confirmed by a return to normal sinus rhythm.

While a particular embodiment of the present invention has been shownand described, modifications may be made. It is therefore intended inthe appended claims to cover all such changes and modifications, whichfall within the true spirit and scope of the invention.

1. A method for delivering a therapy to terminate a tachycardia event,comprising: sensing cardiac signals and detecting the tachycardia eventin response to the sensed cardiac signals; delivering a first pluralityof pacing pulses in response to the detected cardiac event; determininga return cycle length generated subsequent to the delivery of the firstplurality of pacing pulses; determining whether the return cycle lengthis greater than a cycle length associated with the tachycardia event andless than a sum of the cycle length associated with the tachycardiaevent and a total prematurity associated with the first plurality ofpacing pulses; and adjusting delivery of a second plurality of pacingpulses in response to the return cycle length not being greater than thecycle length associated with the tachycardia event and less than the sumof the cycle length associated with the tachycardia event and the totalprematurity associated with the first plurality of pacing pulses.
 2. Themethod of claim 1, further comprising determining a transit timecorresponding to a distance between the electrode and a reentrantcircuit associated with the tachycardia event.
 3. The method of claim 2,wherein the transit time is determined as one half of a differencebetween the return cycle length and the cycle length associated with thetachycardia event.
 4. The method of claim 2, further comprisingdetermining a minimum number of pacing pulses to enable delivery of thesecond plurality of pacing pulses to reach the reentrant circuit inresponse to the determined transit time.
 5. The method of claim 4,wherein the second plurality of pacing pulses is equal to one more thanthe determined minimum number of pacing pulses.
 6. The method of claim4, further comprising determining prematurities corresponding to eachpacing pulse of the second plurality of pacing pulses, wherein the totalprematurity corresponds to a sum of the determined prematurities, andthe minimum number of pacing pulses is determined as a ratio of thetransit time and the prematurities.
 7. The method of claim 6, whereinthe ratio corresponds to twice the transit time relative to theprematurities.
 8. The method of claim 6, wherein determiningprematurities comprises: determining a cycle length of each pacing pulseof the second plurality of pacing pulses; and determining a differencebetween the cycle length of each pacing pulse of the second plurality ofpacing pulses and the cycle length associated with the tachycardiaevent.
 9. The method of claim 8, further comprising increasing thedetermined prematurity of at least one pacing pulse of the secondplurality of pacing pulses.
 10. The method of claim 8, furthercomprising increasing the determined prematurity of a last pacing pulseof the second plurality of pacing pulses.
 11. A medical device fordelivering a therapy to terminate a tachycardia event, comprising: anelectrode to sense cardiac signals and to deliver the therapy, thetherapy including a first plurality of pacing pulses; sensing circuitry,electrically coupled to the electrode, to detect the tachycardia eventin response to the sensed cardiac signals; and a processor to controldelivery of the therapy, wherein the processor determines whether areturn cycle length generated subsequent to the delivery of the firstplurality of pacing pulses is greater than a cycle length associatedwith the tachycardia event and less than a sum of the cycle lengthassociated with the tachycardia event and a total prematurity associatedwith the first plurality of pacing pulses, and adjusts delivery of asecond plurality of pacing pulses in response to the return cycle lengthnot being greater than the cycle length associated with the tachycardiaevent and less than the sum of the cycle length associated with thetachycardia event and the total prematurity associated with the firstplurality of pacing pulses.
 12. The device of claim 11, wherein theprocessor determines a transit time corresponding to a distance betweenthe electrode and a reentrant circuit associated with the tachycardiaevent.
 13. The device of claim 12, wherein the transit time isdetermined as one half of a difference between the return cycle lengthand the cycle length associated with the tachycardia event.
 14. Thedevice of claim 12, wherein the processor determines a minimum number ofpacing pulses to enable delivery of the second plurality of pacingpulses to reach the reentrant circuit in response to the determinedtransit time.
 15. The device of claim 14, wherein the second pluralityof pacing pulses is equal to one more than the determined minimum numberof pacing pulses.
 16. The device of claim 14, wherein the processordetermines prematurities corresponding to each pacing pulse of thesecond plurality of pacing pulses, wherein the total prematuritycorresponds to a sum of the determined prematurities, and the minimumnumber of pacing pulses is determined as a ratio of the transit time andthe prematurities.
 17. The device of claim 16, wherein the ratiocorresponds to twice the transit time relative to the prematurities. 18.The device of claim 16, wherein the processor determines a cycle lengthof each pacing pulse of the second plurality of pacing pulses, anddetermines a difference between the cycle length of each pacing pulse ofthe second plurality of pacing pulses and the cycle length associatedwith the tachycardia event as the prematurities.
 19. The device of claim18, wherein the processor increases the determined prematurity of atleast one pacing pulse of the second plurality of pacing pulses.
 20. Thedevice of claim 18, wherein the processor increases the determinedprematurity of a last pacing pulse of the second plurality of pacingpulses.